1. Field of the Invention
The present invention relates to a method and apparatus for determining timing for initial ranging of user equipment by using ranging of adjacent pieces of user equipment in a multi-hop mobile relay (MMR) system, and more particularly, to a method and apparatus for determining timing for initial ranging of user equipment in which a power value and a timing value for periodical ranging of adjacent pieces of user equipment are measured so as to minimize an uplink timing error of a base station. That is, the present invention relates to a method of properly selecting timing for initial ranging of user equipment in the MMR system and a computer-readable medium having embodied thereon a computer program for executing the method.
This work was supported by the IT R&D program of MIC/IITA. [2006-S-011-01, Development of relay/mesh communication system for multi-hop WiBro]
2. Description of the Related Art
In multi-hop mobile relay (MMR) systems, four ranging modes are defined which include initial ranging that initially uses a code division multiple access (CDMA) code and sends it to a base station in order for user equipment to attempt a call entry, periodic ranging that periodically reports a state of the user equipment to the base station after a call process is complete, bandwidth request ranging that allows the user equipment to request an uplink bandwidth to a scheduler of the base station, and handover ranging that attempts a handover.
In all communication systems having a base station that operates on the basis of a predetermined reference timing value, a time required for a signal that is transmitted from user equipment to reach the base station is estimated so as to regulate transmission timing for each piece of user equipment, and as a result, signals from all pieces of user equipment are received by the base station at the same timing. In order to achieve the aforementioned purpose when a piece of user equipment newly accesses a system, a series of processes are performed to measure a delayed time when a signal is transferred to the base station, which is called as an initial ranging process. Timing for an initial ranging signal does not coincide with timings for signals of other pieces of user equipment which have undergone the initial ranging process. Thus, the initial ranging signal may interfere with signals of other pieces of user equipment.
In general, while not knowing an exact uplink start point of the base station, user equipment that initially enters a call transmits an initial ranging code by using an initial timing value and a power value which are arbitrarily stored. After the initial ranging code is received, the base station measures the timing value and the power value of the initial ranging code and then transmits a response message of the initial ranging code to a next frame. In this case, a parameter of the response message broadcasts a modified timing value that can be adjusted according to uplink synchronization of the base station along with the received power value to the user equipment within a cell.
As described above, since the initial power value and the timing value are used in the initial ranging process without alteration, if the user equipment attempts the initial ranging at the same time as when other pieces of user equipment attempt periodic ranging and handover ranging, an error occurs between transmission timing of the initial ranging and uplink timing, thereby acting as inter-symbol interference (ISI) and inter-channel interference (ICI) of different ranging. As a result, the initial ranging acts as an interference signal in a ranging process of other pieces of user equipment. Accordingly, there is a problem in that a receiving rate of the base station is reduced with respect to a ranging signal of other pieces of user equipment.